AC/DC CONVERTER AND AC/DC CONVERTING METHOD
An AC/DC converter includes: a rectifier circuit that rectifies an alternating current input; a PFC circuit that has an inductance element, a switching element, and a diode and steps up and outputs an output of the rectifier circuit; a DC/DC converting circuit that converts an output of the PFC circuit into a direct current voltage power source of a predetermined voltage value; a target voltage command generating circuit that designates the target voltage so that the target voltage is low in a region; a target voltage converting circuit that generates a converted target voltage by converting the target voltage designated by the target voltage command generating circuit so that the target voltage is changed in a first period; and a voltage command generating circuit that generates an on and off control signal for controlling on and off of the switching element of the PFC circuit.
This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2013-262929, filed on Dec. 19, 2013, the entire contents of which are incorporated herein by reference.
FIELDThe embodiments discussed herein are related to an AC/DC converter and an AC/DC converting method.
BACKGROUNDAn AC/DC converter that converts an alternating current into a direct current is widely used in a power source of an electronic apparatus. In general, an AC/DC converter has to operate at an operating point that achieves as efficient operation as possible in order to keep the running cost of an electronic apparatus low. Furthermore, it is desirable that an AC/DC converter contain fewer harmonics.
In general, a commercial alternating current power source in a range from 100 V to 240 V is inputted to an AC/DC converter, and a direct-current voltage (pulsating current) is formed through full-wave rectification using a diode element. This direct-current voltage is stepped up by switching on and off of a switching element of a power factor correction (PFC) circuit. The voltage thus stepped up is once converted from a direct current into an alternating current by switching on and off of a switching element of a single-ended forward converter in a DC/DC converting circuit, is stepped down by inputting the alternating current to an isolation transformer, and is then converted into a direct current by a rectifier circuit. In this way, the voltage is finally converted into a direct current of a low voltage in a range from 12 V to 48 V.
For example, a CPU is switched between a high-load processing state and an idling state in order to achieve an energy-saving function in a server apparatus. There are cases where a power source unit mounted in the apparatus is switched on or off accordingly. When such an operation occurs, the current consumed markedly fluctuates. This influences the voltage, and the voltage fluctuation also becomes large. Such a large voltage fluctuation causes malfunction of the apparatus. Therefore, the voltage fluctuation of the power source has to be reduced in order to achieve stable operation of the server apparatus.
In general, when the response speed of a power source circuit is high, the voltage fluctuation is small, whereas when the response speed is low, the voltage fluctuation is large.
The main causes of loss in a PFC circuit are classified into switching loss of an FET etc. and resistive loss of a diode, an FET, a choke coil, etc. Although the ratio between these losses varies depending on the specification of the circuit, the percentage of the switching loss is larger in a region where an output electric current is low, and the percentage of the resistive loss is larger in a region where an output electric current is high. Accordingly, in the region where an output electric current is low, reducing an output voltage of the PFC circuit (reducing a step-up ratio), which reduces the switching loss, achieves higher conversion efficiency. Meanwhile, in the region where an output electric current is high, increasing the output voltage of the PFC circuit, which reduces the electric current and the resistive loss, is more efficient. Because of this, conventionally, an output voltage of a PFC circuit is set to one that achieves high efficiency in accordance with an electric current value requested for a system.
In order to further improve the efficiency of an AC/DC converter, a technique of switching an output voltage of a PFC circuit in accordance with a load (output electric current) is proposed. In such an AC/DC converter, an output electric current of a DC/DC converting circuit is detected, and a target voltage is designated so that an output voltage of a PFC circuit is low in a region where the output electric current is low and the output voltage of the PFC circuit is high in a region where the output electric current is high. Then, on and off of an FET of the PFC circuit is controlled so that the output voltage of the PFC circuit becomes the target voltage. As a result, the PFC circuit outputs an output voltage in accordance with the target voltage, and a high efficiency is achieved throughout a wide load (output electric current) value range.
However, in this AC/DC converter, the output voltage of the PFC circuit is decreased when a low output electric current is detected, and the output voltage of the PFC circuit is increased when a high output electric current is detected. Therefore, in a case where the output electric current fluctuates over a short cycle, the output voltage of the PFC circuit fluctuates in the middle of the transition of the output voltage from a high voltage to a low voltage or vice versa. Consequently, the output voltage of the PFC circuit is undesirably not stable.
In view of this, a technique has been proposed for delaying switching of the PFC from on to off by turning on the PFC circuit and thereby increasing the output voltage of the PFC circuit in a case where a load (output) is high and by turning off the PFC circuit and thereby decreasing the output voltage of the PFC circuit in a case where the load (output electric current) is low.
In general, in order to increase the response speed, it is desirable that the output voltage of the PFC circuit be high. Meanwhile, in order to increase the efficiency, it is desirable that the output voltage of the PFC circuit be low in a region where a load (output electric current) is low.
In the arrangement in which the PFC control circuit is turned on or off, when the output electric current becomes low, the PFC circuit is stopped, that is, the output voltage of the PFC circuit is decreased. Accordingly, the response speed is low, and when an electric current fluctuation occurs, a voltage fluctuation undesirably becomes large. In addition, when the output electric current becomes large again, the PFC circuit is activated, but it takes a certain time to activate the PFC circuit. In this case, the PFC circuit is under no control for the certain time. Accordingly, the response speed becomes low, and when an electric current fluctuation occurs, the voltage fluctuation becomes large.
The following is reference documents:
[Document 1] Japanese Laid-open Patent Publication No. 2009-261042 and
[Document 2] International Publication Pamphlet No. WO2004/059822.
SUMMARYAccording to an aspect of the invention, an AC/DC converter includes: a rectifier circuit that rectifies an alternating current input; a PFC circuit that has an inductance element, a switching element, and a diode and steps up and outputs an output of the rectifier circuit in accordance with a target voltage by controlling on and off of the switching element; a DC/DC converting circuit that converts an output of the PFC circuit into a direct current voltage power source of a predetermined voltage value; a target voltage command generating circuit that designates the target voltage so that the target voltage is low in a region where an output electric current of the DC/DC converting circuit is low and so that the target voltage is high in a region where the output electric current of the DC/DC converting circuit is high; a target voltage converting circuit that generates a converted target voltage by converting the target voltage designated by the target voltage command generating circuit so that the target voltage is changed in a first period when the output electric current of the DC/DC converting circuit is shifted from low to high and so that the target voltage is changed in a second period longer than the first period when the output electric current of the DC/DC converting circuit is shifted from high to low; and a voltage command generating circuit that generates an on and off control signal for controlling on and off of the switching element of the PFC circuit in accordance with the converted target voltage.
The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.
A general AC/DC converter is described before describing embodiments.
For example, an AC/DC converter 10 receives input of an alternating current (AC) from a 100 V to 240 V commercial power source 1, converts the alternating current into a direct current, steps up the direct current, converts the direct current into a predetermined direct current voltage in a range from 12 V to 48 V (DC/DC conversion), and then outputs the direct current voltage to a load (for example, information apparatus) 3.
As illustrated in
The rectifier circuit 11 has four diode elements that are connected to each other. This forms a full-wave rectification waveform of a direct current voltage. The PFC circuit 12 steps up the direct current voltage outputted from the rectifier circuit 11 by switching on or off a switching element (FET) Q1. For example, the PFC circuit 12 steps up the direct current voltage to a voltage in a range from DC 350 V to 400 V. A voltage command generating circuit 50 generates a voltage command that switches on or off the switching element Q1 in accordance with the output voltage of the PFC circuit 12 (a terminal voltage of a capacitor Cb in
The DC/DC converting circuit 13 receives the voltage stepped up by the PFC circuit 12, converts the direct current into an alternating current once by switching on or off a switching element Qp1 of a single-ended forward converter, and then outputs the alternating current to an insulation transformer T. The voltage stepped down by the transformer T is converted into a direct current by a rectifier circuit made up of a diode D6 and a capacitor Co so as to be finally converted into a direct current of a low voltage in a range from 12 V to 48 V, and is then outputted to the load 3. An output voltage detection circuit 15 detects an output voltage Vout, and a DC/DC section control circuit 14 switches on or off a switching element Qp1 so that the output voltage Vout becomes a predetermined voltage.
As illustrated in
During a period t2 to t3, when the output electric current IO rapidly decreases from 50% to 0%, the output voltage VPFC of the PFC circuit 12 increases and reaches an overvoltage detection point of the PFC circuit 12. When the output voltage VPFC of the PFC circuit 12 reaches the overvoltage detection point, feedback is saturated, and the voltage becomes invariable. During a period t3 to t4, the output electric current IO increases again from 0% to 50%, and the output voltage VPFC of the PFC circuit 12 decreases as in the case of t2.
As described above, the PFC circuit 12 described with reference to
The main causes of loss in the PFC circuit 12 are classified into switching loss of a diode D5 and Q1 (FET), etc. and resistive loss of the diode D5, Q1 (FET), a choke coil L1, etc. Although the ratio between these losses varies depending on the specification of the circuit, the percentage of the switching loss is larger in a region where an output electric current is low, and the percentage of the resistive loss is larger in a region where an output electric current is high. Accordingly, in the region where an output electric current is low, reducing an output voltage (reducing a step-up ratio), which reduces the switching loss, achieves a higher conversion efficiency. Meanwhile, in the region where an output electric current is high, increasing an output voltage, which reduces the electric current and the resistive loss, is more efficient. Because of this, conventionally, the output voltage of the PFC circuit 12, that is, the reference potential Vref is set to one that achieves high efficiency in accordance with an electric current value requested for a system.
The characteristic of conversion efficiency of the PFC circuit 12 varies depending on the output electric current. As indicated by the solid line in
However, according to the circuit of
The load on the server apparatus fluctuates on the order of (approximately) several μsec to several hundreds of μsec in the case of semiconductor parts, and stable operation of the server apparatus on the order of several msec or more is demanded. Because of these, it is important to reduce a voltage fluctuation of a PFC circuit and a DC/DC converting circuit in an AC/DC converter and a DC/DC converting circuit in a server apparatus.
In view of this, a PFC circuit is proposed which achieves high conversion efficiency even in a region where the output electric current is low and a region where the output electric current is high by performing a control of switching a setting voltage at an intersection of the 370 V efficiency curve and the 400 V efficiency curve illustrated in
A PFC circuit has the following problem. Specifically, when an output voltage of a PFC circuit is low, response speed to an output electric current fluctuation becomes low, and therefore a voltage fluctuation becomes large. This deteriorates the stability of the circuit. This problem is discussed below by taking, as an example, a PFC circuit having the efficiency characteristic as illustrated in
Meanwhile, the response speed is low in the case of 370 V, and the response speed in the case of 400 V is higher than that in the case of 370 V. When the response is slow, a fluctuation of the output voltage VFCf of the PFC circuit 12 becomes large upon occurrence of a rapid load fluctuation in the PFC circuit 12.
As in the case of
When the output voltage VPFC of the PFC circuit 12 fluctuates, an output voltage VO of the DC/DC converting circuit 13 that is located in a stage following the PFC circuit 12 also fluctuates. Furthermore, in a case where the amount of fluctuation of the output voltage VPFC of the PFC circuit 12 is large, the amount of fluctuation of the output voltage VO of the DC/DC converting circuit 13 also becomes large. In general, output voltage accuracy (stability) is one element of the specification of an AC/DC converter. In order to satisfy this element of the specification concerning the output voltage accuracy, it is important to reduce a fluctuation of an output voltage even when a rapid electric current fluctuation occurs.
Furthermore, there has been a proposal of an AC/DC converter that uses another control method of changing an output voltage of a PFC circuit.
Also in the AC/DC converter of
In this case, the period setting circuit 41 sets a period in accordance with a direction of a change of the load state. A PFC on and off switching circuit 42 controls, in accordance with the information on the load state transmitted via the period setting circuit 41, whether to turn on or off a PFC section control circuit 25. The PFC section control circuit 25 has an identical function to the voltage command generating circuit 50 of
In this way, in the AC/DC converter of
As described above, an output voltage of a PFC circuit is increased in order to increase the response speed. Meanwhile, an output voltage of the PFC circuit is lowered in a region where an electric current is low in order to increase efficiency. According to the AC/DC converter of
Furthermore, according to the AC/DC converter of
In the embodiments described below, an AC/DC converter that solves the above problems is disclosed. This AC/DC converter is an efficient AC/DC converter that outputs a stable voltage even at the occurrence of a fluctuation of a load (output electric current).
In the AC/DC converter according to the first embodiment, the output electric current detection circuit 52 detects an output electric current IO of the DC/DC converting circuit 13, and the target voltage command generating circuit 51 outputs a target voltage Vref of the PFC circuit 12 according to the output electric current IO thus detected, as in the circuit of
The target voltage converting circuit 53 converts the target voltage Vref outputted from the target voltage generating circuit 51 into a converted target voltage Vref′. As in
As illustrated in
When the target voltage Vref is low (=370 V), the non-inverting input, the inverting input and the output of the amplifier AMP and the one terminal of the capacitor C1 are at a low level. When the target voltage Vref becomes high (=400 V) in this state, the non-inverting input of the amplifier AMP becomes high. In response to this, an electric current flows from the amplifier AMP to the capacitor C1 via the diode D11 and the resistor R1 that are serially connected to each other. This charges the capacitor C1 and causes the inverting input and the output of the amplifier AMP and the one terminal of the capacitor C1 to be high (=400 V). Since the resistance of the resistor R1 is small, the electric potential of the one terminal of the capacitor C1 rapidly increases in a short time.
Meanwhile, when the target voltage Vref is high (=400 V), the non-inverting input, the inverting input and the output of the amplifier AMP and the one terminal of the capacitor C1 are at a high level. When the target voltage Vref becomes low (=370 V) in this state, the non-inverting input of the amplifier AMP becomes low. In response to this, an electric current flows from the capacitor C1 to the amplifier AMP via the diode D12 and the resistor R2 that are serially connected to each other. This discharges the capacitor C1 and causes the inverting input and the output of the amplifier AMP and the one terminal of the capacitor C1 to be low (=370 V). Since the resistance of the resistor R2 is large, the electric potential of the one terminal of the capacitor C2 gradually decreases over a long time.
As described above, the target voltage converting circuit 53 rapidly increases the converted target voltage Vref′ when the target voltage Vref outputted from the target voltage generating circuit 51 becomes high, whereas the target voltage converting circuit 53 gradually decreases the converted target voltage Vref′ when the target voltage Vref becomes low. In this example, the capacitor C1 functions as a capacitive element that holds the last converted target voltage, and the amplifier AMP whose inverting input and output are connected to each other functions as a charging and discharging circuit that charges or discharges the capacitor C1 in accordance with a difference between the target voltage and the voltage held by the capacitor C1. The diode D11 and the resistor R1 that are serially connected to each other function as a charging pathway connected between the capacitive element and the charging and discharging circuit, and the diode D12 and the resistor R2 that are serially connected to each other function as a discharging pathway connected between the capacitive element and the charging and discharging circuit.
In a case where the output electric current IO changes as indicated by the electric current waveform in the topmost row of
Therefore, when the target voltage Vref shifts from L to H, this shift is instantly reflected in generation of a voltage command in the voltage command generating circuit 50, and the duty of the voltage command increases. Meanwhile, when the target voltage Vref shifts from H to L, this shift is gradually reflected in generation of a voltage command in the voltage command generating circuit 50, and the duty of the voltage command decreases. Even in this case, the PFC circuit 12 continues to operate and be controlled, as indicated by the third row from the top in
In the first embodiment, since the PFC circuit is not stopped even when the output electric current becomes low, it is possible to continue a controllable state. Furthermore, since the target voltage Vref is gradually decreased in a case where the output electric current decreases, response of the PFC circuit is also delayed. Accordingly, even in a case where the output electric current shifts from H to L and the target voltage instantly shifts from L to H, the output voltage of the PFC circuit does not decrease yet, and therefore the response speed remains high. Consequently, a voltage fluctuation becomes small.
Meanwhile, in a case where the output electric current shifts from L to H and the target voltage shifts from H to L, this change is instantly reflected in a voltage command, and the output voltage of the PFC circuit instantly increases, and therefore the response speed instantly becomes high. Consequently, a voltage fluctuation is small.
In the AC/DC converter circuit illustrated in
As has been described above, in the AC/DC converter according to the first embodiment, the PFC circuit is not stopped even in a case where the output electric current decreases. This makes it possible to continue controlling a voltage.
Furthermore, when a decrease of the output electric current is detected, the output voltage of the PFC circuit is decreased although the decrease of the output voltage is gradual. This achieves higher efficiency during a period of time in which the output voltage of the PFC circuit is decreased.
Furthermore, in a case where the output electric current decreases, the output voltage of the PFC circuit is gradually decreased. Therefore, even in a case where an electric current fluctuation occurs over a short cycle as illustrated in
In the AC/DC converter according to the first embodiment, each of the voltage command generating circuit 50, the target voltage command generating circuit 51, and the target voltage converting circuit 53 is realized by an analog circuit, but all of or part of these circuits may be realized by a digital processing circuit. In an AC/DC converter according to the second embodiment described below, a target voltage command generating circuit 51 and a target voltage converting circuit 53 are realized by an analog circuit.
The A/D converter 61 converts an output electric current IO detected by an output electric current detection circuit 52 into a digital signal. The computer 62 is, for example, realized by a microcomputer and performs, based on data of the output electric current IO outputted by the A/D converter 61, digital processing corresponding to the analog processing of the target voltage command generating circuit 51 and the target voltage converting circuit 53 in the first embodiment. That is, a target voltage Vref is calculated from the data of the output electric current IO, and a converted target voltage Vref′ is generated in accordance with a direction of a change of the output electric current IO. The D/A converter 63 converts the converted target voltage Vref′ into an analog signal, and outputs the analog signal to the voltage command generating circuit 50. Further explanation is omitted since it is easy for a person skilled in the art to perform digital processing instead of the analog processing described in the first embodiment. The second embodiment produces similar effects to those of the first embodiment.
All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
Claims
1. An AC/DC converter comprising:
- a rectifier circuit that rectifies an alternating current input;
- a PFC circuit that has an inductance element, a switching element, and a diode and steps up and outputs an output of the rectifier circuit in accordance with a target voltage by controlling on and off of the switching element;
- a DC/DC converting circuit that converts an output of the PFC circuit into a direct current voltage power source of a predetermined voltage value;
- a target voltage command generating circuit that designates the target voltage so that the target voltage is low in a region where an output electric current of the DC/DC converting circuit is low and so that the target voltage is high in a region where the output electric current of the DC/DC converting circuit is high;
- a target voltage converting circuit that generates a converted target voltage by converting the target voltage designated by the target voltage command generating circuit so that the target voltage is changed in a first period when the output electric current of the DC/DC converting circuit is shifted from low to high and so that the target voltage is changed in a second period longer than the first period when the output electric current of the DC/DC converting circuit is shifted from high to low; and
- a voltage command generating circuit that generates an on and off control signal for controlling on and off of the switching element of the PFC circuit in accordance with the converted target voltage.
2. The AC/DC converter according to claim 1, wherein:
- the target voltage converting circuit includes:
- a capacitive element that holds the last converted target voltage;
- a charging and discharging circuit that charges or discharges the capacitive element in accordance with a difference between the target voltage designated by the target voltage command generating circuit and the voltage held by the capacitive element;
- a charging pathway coupled between the capacitive element and the charging and discharging circuit; and
- a discharging pathway coupled between the capacitive element and the charging and discharging circuit;
- wherein resistance of the charging pathway is smaller than resistance of the discharging pathway.
3. The AC/DC converter according to claim 2, wherein:
- the charging pathway has a first diode and a first resistor that are serially coupled to each other;
- the first diode is coupled in a forward direction from the charging and discharging circuit toward the capacitive element;
- the discharging pathway has a second diode and a second resistor that are serially coupled to each other;
- the second diode is coupled in a reverse direction from the capacitive element toward the charging and discharging circuit; and
- resistance of the second resistor is larger than resistance of the first resistor.
4. An AC/DC converting method for rectifying an alternating current input, stepping up a rectified voltage in accordance with a target voltage, performing DC/DC conversion of the rectified voltage thus stepped up into a direct current voltage of a predetermined voltage value, and then outputting the direct current voltage, comprising:
- detecting a DC/DC converted output electric current;
- designating the target voltage so that the target voltage is low in a region where the output electric current thus detected is low and so that the target voltage is high in a region where the output electric current thus detected is high;
- generating a converted target voltage by converting the target voltage so that the target voltage is changed in a first period when the output electric current is shifted from low to high and so that the target voltage is changed in a second period longer than the first period when the output electric current is shifted from high to low; and
- controlling stepping-up of the rectified voltage in accordance with the converted target voltage.
Type: Application
Filed: Dec 9, 2014
Publication Date: Jun 25, 2015
Inventors: Yukio YOSHINO (Tokorozawa), Hiroshi SHIMAMORI (Yokosuka)
Application Number: 14/564,402